This invention relates to a device for controlling the output frequency of an inverter over a wide range.
Known is an inverter apparatus adapted to control the operating speed of an induction motor over a wide range. One example of such apparatus is shown in FIG. 1. In the example, a three phase inverter 2 of the transistor type is connected to a DC power source 1. The output of the inverter 2 is connected to a three-phase induction motor 6 having a rotor directly coupled with a tachometer 7. Current detectors 3, 4, and 5 are further provided for detecting three phase AC currents delivered from the inverter 2 which is controlled in PWM (pulse width modulating) mode as described hereinafter.
The control circuit of the inverter comprises a speed setting device 8 which delivers a reference signal S.sub.R which is compared with the output voltage V.sub.S from the tachometer 7, and the difference is amplified by an amplifier 9, thereby providing a reference signal T* representing a torque required for reducing the difference into zero. The torque reference signal T* is applied to a function generator 10 to provide a reference signal id* which is then applied to a three phase AC current reference circuit 13.
The torque reference signal T* is further applied to a potentiometer 11 to provide a voltage signal S.sub.V which is added to the output voltage V.sub.S from the tachometer 7, and the result thus obtained is applied to a voltage/frequency (V/F) converter 12 for providing a frequency reference signal fo*. The frequency reference signal fo* is also applied to the three-phase AC current references circuit 13.
Upon reception of the current reference signal id* and the frequency reference signal fo*, the current reference circuit 13 delivers sinusoidal three phase current reference signals i.sub.U *, i.sub.V * and i.sub.W * to be used for controlling the inverter 2. That is, the reference signals i.sub.U *, i.sub.V * and i.sub.W * are then compared in differential amplifiers 21.sub.U, 21.sub.V and 21.sub.W with the outputs i.sub.U, i.sub.V and i.sub.W from the current detectors 3, 4 and 5, respectively, and the compared results are amplified therein to provide three phase AC voltage reference signals V.sub.U *, V.sub.V * and V.sub.W *.
The pulse-width modulating (PWM) control of the inverter 2 is carried out as follows. The voltage reference signals V.sub.U *, V.sub.V *, and V.sub.W * are compared in comparators 22.sub.U, 22.sub.V and 22.sub.W with a triangular output voltage V.sub..DELTA. from a triangular wave generating circuit 14 for obtaining PWM signals which are held, as shown in FIG. 2, at a high level when the reference signals V.sub.U *, V.sub.V *, and V.sub.W * exceed the triangular wave V.sub..DELTA.. The PWM signals are applied through driving circuits 23.sub.U, 23.sub.V and 23.sub.W to the base electrodes of the transistors in the inverter 2. Since the PWM signals applied to the base electrodes of the transistors have waveforms quite similar to the output voltages V.sub.U, V.sub.V, and V.sub.W delivered from the inverter 2 as shown in FIG. 3, the output voltages V.sub.U, V.sub.V, and V.sub.W are shown in FIG. 2 instead of the PWM signals although the maximum amplitudes of the latter voltages are much greater than those of the PWM signals. Furthermore, it should be noted that a phase-to-phase voltage V.sub.U-V delivered from the inverter has a frequency twice of that of the PWM signals.
In the above described control of the inverter apparatus, however, since the three phase currents of the induction motor 6 are controlled with respect to their average values (or entire values) rather than their instantaneous values, it is essential that the frequency response of the control circuit is to be reduced to a value sufficient to reduce inclusion of ripples.
More specifically, when the output frequency of the inverter is asssumed to be 100 Hz and the AC currents are desired to be controlled without remarkable errors, the open gain of the current minor loop should be more than 10, and .omega..sub.c in Bode response of the same loop should be more than 2.pi..times.100.times.10=6000 rad./s. However, since an excessive value of .omega..sub.c increases the content of ripples, the frequency response should be limited to the lowermost value so far as the required currents can be obtained from the inverter. This inevitably reduces the frequency of the PWM control, thus entailing an unstable operation in a high-frequency range of the inverter operation.
It is further known that the operating frequency of an inverter can be increased by using an instantaneous control circuit as shown in FIG. 4. In this circuit, the amplifiers 21.sub.U, 21.sub.V and 21.sub.W, comparators 22.sub.U, 22.sub.V and 22.sub.W, and the triangular wave generating circuit 14 shown in FIG. 1 are substituted by comparators 24.sub.U, 24.sub.V and 24.sub.W each having a hysteresis characteristic. The comparators 24.sub.U, 24.sub.V and 24.sub.W each having a detailed construction as will be described hereinafter compare the outputs i.sub.U, i.sub.V and i.sub.W obtained from the current detectors 3, 4 and 5 with the current reference signals i.sub.U *, i.sub.V * and i.sub.W *, respectively, and deliver the outputs in accordance with the hysteresis characteristics. It is known that the instantaneous control as described above theoretically has no upper limit in its frequency response.
An output current i.sub.u delivered from the inverter 2 under the above described instantaneous control, and a phase-to-phase voltage V.sub.U-V thus obtained are shown in FIGS. 5(a) and 5(b) respectively. In this control, the output current i.sub.U delivered from the inverter is compared in the comparator 24.sub.U with the current reference signal i.sub.U *, and the difference is controlled within a variation range corresponding to the hysteresis characteristic of the comparator 24.sub.U. The phase-to-phase voltage U.sub.U-V is thus made up of a bidirectional pulse swinging above and below the zero voltage level.
On the other hand, the output current i.sub.U and the phase-to-phase voltage V.sub.U-V in the case of the PWM control are shown in FIGS. 6(a) and 6(b). Since the output voltages of the inverter are synchronized with each other by the triangular wave, the phase-to-phase voltage V.sub.U-V in one half cycle is made up of a unidirectional pulse swinging in one direction from the zero voltage level.
For this reason, the current variation rate in the output currents obtained by the PWM control tends to reduce particularly in a low output voltage range of the inverter.
In contrast, the current variation rate in the output currents obtained by the instantaneous control is great because of the bidirectional pulse forming the phase-to-phase voltage V.sub.U-V as described above. Thus, a difficulty of enhancing ripples contained in the output current and increasing magnetic noises in the induction motor is thereby caused, particularly in a low-frequency operation of the inverter.